I don't put any liquid at all in with a pork shoulder roast. It seems to have quite enough of its own. I've never done a pork rib roast in the crockpot, so I don't know how much that would take.

When I was working and had to put things on early I used to put a frozen roast into the removable crockpot crock, and then put the whole thing in the microwave on high for about 5 minutes. Then I would take it out of the microwave and put it into the crockpot and add seasonings and so forth. Five minutes in the microwave was enough to thaw out the outer part of the roast, so that it wouldn't be so cold as to crack the ceramic pot.